Part of the xerography copying process requires a high voltage AC power supply provided by a switch mode power converter. Typically, a high voltage quasi-square waveform is generated using push-pull circuitry and then filtered by an inductor-capacitor low pass filter network (i.e., 500 Hz); U.S. Pat. No. 4,714,978 is an example. The resultant waveform is a distorted sinusoid. Usually, the output frequency of the AC converter is limited to around 400 Hz, due to the inherent losses in the xerography process. A pure sinewave is preferred for low noise content. As the duty cycle of the quasi-square waveform is varied, the distorted sinusoid varies in amplitude; unfortunately, the distortion content also varies. The voltage amplitude is varied by control circuitry to keep a regulated output current. A regulated current is preferred to insure uniform copy quality. This is all the more desirable since current is affected by the age of the components, temperature conditions, dirt, etc.
One modern converter which operates over a 50 percent duty cycle is described in Diaz et al U.S. Pat. No. 4,717,994 (and assigned to the assignee of the present invention). The control and operation of conventional switched-mode power supplies is covered in the paper "Conceptually New High-Frequency Switched-Mode Power Amplifier Technique Eliminates Current Ripple", by Cuk and Erickson, Proceedings of POWERCON FIVE, May 4-6, 1978. de Sartre U.S. Pat. Nos. 4,694,386 and Murakami et al U.S. Pat. No. 4,195,335 describe power supplies which provide automatic start-up. Hamilton et al U.S. Pat. No. 3,879,647 describes a converter having a soft start capability. Finally, Sutton U.S. Pat. No. 4,586,119, describes a switching mode power supply which employs current and voltage feedback and sensing.